Issue #16309: Make PYTHONPATH= behavior the same as if PYTHONPATH not set at all.

Thanks to Armin Rigo and Alexey Kachayev.
diff --git a/Misc/ACKS b/Misc/ACKS
index 899df2d..96d8178 100644
--- a/Misc/ACKS
+++ b/Misc/ACKS
@@ -587,6 +587,7 @@
 Andreas Jung
 Tattoo Mabonzo K.
 Bohuslav Kabrda
+Alexey Kachayev
 Bob Kahn
 Kurt B. Kaiser
 Tamito Kajiyama
diff --git a/Misc/NEWS b/Misc/NEWS
index 755e3d1..c6ba38b 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -10,6 +10,9 @@
 Core and Builtins
 -----------------
 
+- Issue #16309: Make PYTHONPATH="" behavior the same as if PYTHONPATH
+  not set at all.
+
 - Issue #10189: Improve the error reporting of SyntaxErrors related to global
   and nonlocal statements.